Method of stabilizing the inductance of an inductor assemblage



July 24, 1956 G- D. SMOLIAR METHOD OF STABILIZING THE] INDUCTANCE OF AN INDUCTOR ASSEMBLAGE Filed April 22, 1954 Percentage Change of Inductance 3 Sheets-Sheet 1 FIG.

ist Temperature 2nd Temperature 3rd Temperature Variation Variation I Variation 1 l l l I I I Delay 6 Microsecondsi I Weight=%Pound l Example l i I I I I lst Cycle 2nd Cycle 1 3rd Cycle I 0 2O 4O 6O 80 I00 I20 M i nu tes INVENTOR GERALD 0. SMOL/AR F G. 6

SL-W ATTORNEY July 24, 1956 G. D. SMOLIAR METHOD OF STABILIZING THE INDUCTANCE OF AN INDUCTOR ASSEMBLAGE Filed April 22, 1954 3 Sheets-Sheet 2 FIG. 4

v G I F //Vl/EA/7OR GERALD D. SMOL/AR ATTOPNEK July 24, 1956 G. D. SMOLIAR METHOD OF STABILIZING THE INI DUCTANCE OF AN INDUCTOR ASSEMBLAGE Filed April 22, 1954 3 Sheqts-Sheet 3 Delay: 62 Microseconds I Weigh1= 2 Pounds Example 2 Minutes FIG. 7

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Minufes A 7' TORNE Y United States Patent METHOD OF STABILIZING THE INDUCTAN CE OF AN INDUCTOR ASSEMBLAGE Gerald D. Smoliar, Brooklyn, N. Y., assignor to Underwood Corporation, New York, N. Y., a corporation of Delaware Application April 22, 1954, Serial No. 424,952

8 Claims. (Cl. 148-121) This invention relates to inductors and more particularly to a method of stabilizing the inductance of an inductor assemblage suitable for use in delay lines of the lumped parameter type.

A lumped parameter delay line, in one form, consists of a plurality of windings in series, with a capacitor connecting a tap on each winding to a common connector.

The portions of the windings between adjacent taps function as inductors. Each inductor with its associated capacitors constitutes one electrical section of the delay line.

The self inductance of each section of the delay line is a function of the number of turns and the dimensions and shape of the coil winding of each inductor, and the permeability of the coil winding core. The mutual inductance between adjacent inductors of each section is determined by the value of their self inductances and the physical relationships of the windings to each other. It has been found that a particular value of mutual inductance between the inductors of adjoining sections will result in a delay line having optimum electrical characteristics, and the value of mutual inductance is relatively critical.

When it is required that the self inductance (hereinafter called the inductance) per section be larger than a few hundred microhenries, the windings may be mounted on magnetic cores.

The magnetic core increases the inductance per section without increasing the number of turns. However, when the coil windings are on a common magnetic core, the resultant mutual inductance between sections is generally much higher than the optimum value, and the electrical characteristics of the delay line are found to be unsatisfactory.

This problem has been solved by a lumped parameter delay line of the type described and claimed in the copending application of Samuel Lubkin, Serial No. 289,236, filed May 22, 1952, and assigned to the same assignee. In this delay line a plurality of cylindrical cores of magnetic material, each having a circularly grooved face and a plane face, are arranged cylindrically on a mounting member with the grooved face of one core adjacent to the plane face of the next core. The cores are separated from each other by non-magnetic spacers. A tapped coil winding is mounted Within the groove of each core. A plurality of capacitors is supported near the cores and each capacitor connects a tap on each winding to a common connecting bus.

The portion of the winding between each pair of adjacent taps is the inductor of the associated electrical section of the delay line. The inductance of each inductor is determined primarily by the thickness of the non-magnetic spacer since the major part of the reluctance in the flux path is the reluctance between the cores.

The total delay of a delay line is a function of the inductance of its inductors and thus the electrical characteristics of a delay line of this type, are determined prin- 2,756,170 Patented July 24, 1956 cipally by the thickness of the spacers between the cores and the positions of the taps on the windings.

As the gap occupied by each spacer is relatively small, (for example, five thousandths of an inch), a small change in the thickness of the spacers will substantially change the inductance and therefore the delay of the delay line. Thus, this type of construction provides a very simple Way of adjusting the delay since the spacers can be changed after the delay line is assembled.

In delay-line applications requiring extremely accurate delays, the delay should not vary by more than one percent after the initial adjustment of the delay line. In some high-precision applications, this stability requirement is even more severe.

Unfortunately, it has been found that normal handling and aging of delay lines produces changes of delay in the order of one to three percent, particularly in the larger dclay lines. This variation appears to be due to a decrease of the length of the gaps between the magnetic cores of each inductor.

Shock treatment, vibration treatment, and increased compression on the spacers do not adequately solve the stability problem.

An object of the invention, therefore, is to provide an improved method of stabilizing the electrical characteristics of an inductor assemblage,

Another object of the invention is to provide an improved method of minimizing the change of delay which can occur as a result of the handling and usage of electrical delay lines.

A further object of the invention is to provide a method of stabilizing the delay of delay lines of the lumped parameter type which employ inductors wound on spaced magnetic cores.

The invention will be described, by way of example only, in connection with the delay line of the lumped parameter type described in the above-cited application.

In accordance with the preferred embodiment of the invention, the inductance of the inductor assemblage is stabilized by heating the inductor assemblage, cooling the inductor assemblage, and repeating the heating and cooling steps.

Other objects and advantages will appear in the subsequent detailed description which is accompanied by a drawing wherein: I

Fig. 1 is a perspective view of a delay line which may be stabilized in accordance with the invention.

Fig. 2 is a perspective view of one part of a delay line section showing the coil winding construction of the delay line illustrated in Fig. 1.

Fig. 3 is an elevational view of the inductor assemblage which includes the inductor of one section of the delay line.

Fig. 4 is a cross sectional view of Fig. 3 along the vertical diameter of the inductor assemblage.

Fig. 5 illustrates the electrical equivalent of a portion of the delay line shown in Fig. 1.

Fig. 6 is a graph illustrating the change in electrical characteristics of a delay line of the type shown in Fig. 1 during the stabilization process in accordance with one embodiment of the invention.

Fig. 7 is a graph showing the percentage change of inductance during the stabilization of another delay line in accordance with another embodiment of the invention.

Fig. 8 is a graph illustrating the percentage change of inductance during the stabilization of still another delay line in accordance with a further embodiment of the invention.

Referring more particularly to Fig. 1, the delay line 1 generally comprises the elongated U-shaped mounting member 2 which supports coil windings positioned in the cores 32 and capacitors 22 mountedbeneath it.

The mounting member 2 comprises a base 5 and two legs 6 and 8. This shape may easily be achieved by milling out a rectangular cross section or channel from a solid block of material. The mounting member 2 may be constructed from a material having good electrical insulating and mechanical properties; for example, a phenolic material having a fabric base to provide mechanical strength.

Mounting holes 17 at each end of the mounting member 2 are used to mount a cover plate (not shown) over the top portion of the delay line. The cover plate also rigidly positions the components mounted on the top of the mounting member 2. A second covering plate may be similarly mounted over the capacitors 22 beneath the mounting member 2 to maintain them in a rigid position.

The capacitors 22 are closely arranged beneath the base 5 of the mounting member 2. The leads of the capacitors 22 extend through holes in the legs 6 and S of the mounting member 2 and may be bent angularly at the exit of the holes in order to support the capacitors 22 in position. The leads function (in place of separate terminal posts) to receive the coil winding leads. The holes are chosen to be of such a diameter that the frictional contact of the leads with the inside surfaces of the holes also assists in supporting the capacitors in position during assembly.

The capacitors 22 are further maintained in position with theiriupper edges in surface-to-surface contact with the lower surface of base 5 by the solder portions 28 and 29 which are utilized for electrical and mechanical connections. The ends of the leads are preferably bent substantially normal to the outer edge of the leg 6 and alternately toward and away from the edge in order to provide mechanical support for connecting bus 31 before soldering. After soldering, the leads may be clipped close to the solder joint. The solder portions together with the leads are of a larger cross section than the diam-' eter of the holes and also function, in conjunction with the bus 31 connecting the bent ends of. the leads together, to prevent any forced withdrawal of the capacitors 22 from the holes. The cores 32 are arranged adjacent to each other between the legs 6 and S of the mounting member 2. The cores 32 each have a circularly grooved portion at one end to receive a coil winding. The leads of the coil windings are connected either to the tie points 60 or to the leads of the capacitors which pass through leg 8.

The cores 32 (known at pot cores) are stacked on a common axis to form an elongated cylindrical assembly and are spaced from each other by spacer sheets 36. The spacer sheets are preferably of mica but other magnetic or non-magnetic materials are also suitable, for example, Teflon.

The cores 32 are arranged between the identical end retaining plates or pieces 38. The end plates 38 are rigidly mounted in the transverse slots 40 which are of equal. depth and are cut into the legs 6 and 8 on lines parallel to the ends of the member 2.

The spring retainer plates 50, arranged between the legs 6 and 8 and adjacent to one end plate 38, are of a spread U-shape. The purpose of this U-shape is to permit the insertion of a suitable tool to compress the spring 49 between the retainer plates in order to insert or remove individual inductor assemblages or mica spacers. Each retainer plate has a rounded boss in the center thereof which functions toretain the spring 49 mounted in position and to prevent lateral movement.

All the components between the legs of the mounting member are held in position by the force exerted due to the compression of the spring 49. In particular, a continuing pressure is maintained on the spacers 36in order to prevent the gap between the pot cores 32 from increasing.

Referring to the core and coil winding assemblage in Fig. 2, a portion of an inductor is shown. The core 32,

made from a material having a high permeability and preferably of a low electrical conductance (a ferrite, for example), has a circularly grooved face and a plane face 82. The coil winding 34 mounted on the hub 84 is cemented in position completely within the groove 83 of the pot core 32.

The tap 70 which is connected to a turn of the coil winding may be conveniently produced by twisting a portion of the wire of the coil winding to a suitable lead length at the appropriate time and position during winding of the coil. The end leads 64 of the winding are arranged near the tap 70 and the coil is mounted with the three leads extending through the opening 86 in one sector of the score 32.

The cores 32 are cylindrically arranged adjacent to each other with the grooved face of one core next to the plane face of the next succeeding core as shown in Fig. 3.

The spacer 36, which is mounted between the cores 32, is preferably of a square shape with the length of each side approximately equal to the diameter, of the cores. If necessary, the spacer may be made slightly larger in order to aid, in eliminating variations in spacing of the cores dueto burrs at the edges of the spacers.

The equivalent of an electrical section of the delay line 1 is shown in Fig. 5 where corresponding electrical parts of the delay line aredesignated by the same reference characters as are utilized in the description of the mechanical construction above.

The coil windings 34 are in series connection, and the capacitors 22 are connected from the taps '70 to the common connection 31. With this arrangement an electrical section of the delay line includes inductor 30 comprising segments of two adjoining coil windings between the taps 70, and half of the capacitance of the capacitors connected to the taps. Therefore, in an electrical sense, the inductor 30 of one. section of the line is made up of contributions of inductance from the adjacent windings. Each capacitor contributes half of its capacitance to each of'the adjoining sections. The end capacitors are chosen to have substantially half the capacity of the intermediate capacitors. Actual calculation indicates that for the usual assumptions the end capacity should be about 0.47 times the capacity. of the intermediate capacitors for best results. The end leads 64 of the adjacent coil windings 34 are connected together at the tie point 60.

Summarizing, each coil winding 34 which comprises a complete tapped winding in a single core, contributes a portion of the inductance to two sections of the delay line.

Therefore, mutual inductance exists between the adjacent sections, and is of an amount which is a function of the position of the tap, since the tap position will determine the ratio of turns between adjacent sections. For example, if the tap is close to one end of the winding,,the mutual inductance is small, but if the tap is near the center of the winding, the mutual inductance is large.

Close control over the inductance of each section is necessary in order to design and manufacture delay lines to particular specifications. Due to variations in the construction of the coil windings and the cores, and to the lack of complete uniformity of the magnetic properties of the core material, it is somewhat difficult to predict accurately the, inductance of each core and coil winding combination. A very convenient and simple way isprovided to adjust for any small variation in the requisite inductances.

Referring to Fig. 4, which illustrates a cross section of the inductor assemblage of Fig. 3, the major portion of the. magnetic circuit of each inductor comprises the continuous. path 90 perpendicular to the turns of and around across section of the coil winding and through the two adjacent cores. That is, the back of the. next succeeding core provides a portion of the flux path.

Since thecore material has a relatively high permeability, the major-part of the reluctance in the flux path is in'the gap-betweenthe hub 84 and the back of the adjacent core occupied by-the non-magnetic spacer 36. If the gap is relatively small, (for example, five thousandths of an inch), a small change in the gap will substantially alfect the inductance.

til a proper temperature is reached as indicated by a thermometer or other temperature indicator.

The delay line may be cooled outside the oven until about room temperature is reached. Forced cooling by rafter the delay line is assembled the inductance is 5 fan or blower may be employed to increase the rate of easily adjusted by varying the gap between cores by cooling. the simple expedient of inserting spacers of varying thick- 4 EXAMPLE 1 F between the Inductors Thus mmor dlgerenqes m Referring to the graph in Fig. 6, the percentage change mductance may be compensated for, and uniformity of inductance ma be obtained 10 of mductance of a delay llne having a delay of SIX m1- y croseconds and a weight of of a pound is illustrated. It has been noted that after handling and usage of The delay line 1s heat cycled three tlmes, each cycle a delay l1ne its inductance tends to increase. Th1s has takin about fort minutes been attributed to small decreases in the size of the I g y n this example the temperature 1s varled durmg each gaps between the cores due to setting. The increase cycle through about one hundred sixty degrees, that 1s, m inductance causes a corresponding increase in delay. f o

mm centlgrade (around room temperature) to about In many delay line applications, the change 1n delay 5 centigrade and back to 20 centlgrade. About does not aflfect the operation of the circuitry. However, t

wenty minutes are required during the mcreaslng temwhere extremely precise delays are required, it 1s necesperature port1on of the cycle and about twenty minutes sary that the delay line be permanently set before usage are required dunng the coolmg portion of the cycle. so that further delay changes W111 not exceed the toler- 20 Th e percentage change of mductance after the first cycle ance requlrements of the circuitry.

. is about 1.6 percent. Cooling 1s preferably done by It has been d1SCOV616d that the delay of a delay 11116. forcin air over the dela line of the lumped parameter type can be permanently set Af g y ter the delay line has cooled down to room temand therefore stabilized by repeatedly varying the temperature it is again heated and cooled la a simllar manperature of the delay line.

. ner. The second heat cycle produces a change of about More particularly, and in accordance with a preferred 6 t ki t t 1 h th d f 2 2 embodiment of the invention, the inductance of each Percen ma mg a o a c ange m e or er 0 inductor assemblage can be increased to a final inductpercent ance value which is extremely stable by repeatedly heat- T heat cycle 15 repeated a thud f After the ing and cooling the inductor assemblage thlrd heat cycle only a small change of mductance oc- This phenomenon is illustrated by Table I which shows 0111's indicating that t1}?! f y line Stabililadthe percentage change of the inductance of a sample of Thefinal change in mductance 1s about .1 percent five inductor assemblages. making a total inductance variation from before sta- TABLE I Percentage change of inductance Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Heat Cycle Change Total Change Total Change Total Change Total Change Total per Change per Change per Change per Change per Change Cycle Cycle Cycle Cycle Cycle In all cores the total inductance change is between bilization to after stabilization of about 2.3 percent. plus one percent and plus three percent. In general, Since the total percentage change of delay is one-half as can be seen from Table I, increasing the number of the total percentage change of inductance (for small heat cycles tends to increase the inductor stability. From changes), the delay increases about 1.15 percent. the standpointof engineering economy three cycles are The delay line of Example 1 is similar in size and sufl'icientsincethe fourth cycle only shows an average construction to the delay line 1 illustrated in Fig. 1. increase of +0.13 percent. However, even after as many It should be noted that the spring which maintains as five cycles, an average instability of +0.04 percent the inductor assemblages in position should be strong remains. enough to take up the decrease in gap spacing during It has also been found that the larger the gap size the the stabilization process.

smaller the change in inductance after stab1l1zat1on by EXAMPLE 2 heat cycling.

The significant factors appear to be the repeated change Referring to the graph of Fig. 7, the stabilization of in temperature and the amount of each variation rather a delay line having a delay of 6.2 microseconds and a than the direction of change. The amount of each variweight of two pounds is illustrated. Because of the ation is not critical. However, as will be more readily weight about sixt minutes are re uired durin which Y q g apparent hereinafter, a single heating and cooling cycle to heat the delay line from room temperature to about 90 produces substantial stabilization. centigrade and thirty minutes are required to cool the The following examples show three typical applicadelay line back to room temperature making a total temtions of the invention to delay line stabilization. perature variation through one hundred forty degrees per' It should be noted that the temperatures indicated cycle. Cooling is usually accelerated by forcing air over are not critical and are given by way of example only. the delay line. Temperature variations per cycle in the range of one During the first cycle of the stabilization process thehundred to two hundred fifty degrees centigrade are suitdelay line changes inductance by about 1.6 percent. The: able. percentage change of inductance is increased to about 1.9

The delay line may be heated in a suitable oven unpercent after the second cycle and levels olf at about 2.0

7: percent after the: third, cycle; The-- total stabilization process takes-about 270-minutes fora delay line of this type: i

Referring to the graph in Fig. 8' thepercentage change of inductance during the stabilization of a delay line having a delay of 380 microseconds and a weight of 3% pounds is shown; Because ofltbe. weight each cycle lasts. about 135 minutes, 90 minutes.beingmequiredto heat the. delay line from room temperature to about 120 centigrade and about .45 minutes being required to cool the delay line back to room temperature by forced draft cooling, Thus, the totaltemperature: variationiper. cycle is. in the order'of two hundred. degrees;

At the end. of. the. first cycle the percentage change ofinductance. is in the order'of'1.75 percent- At the end of the second cycle thechangehas increased to about 2.25. percent, and stabilizes; at about.2.3 percent at' the end of the. third cycle.. Theeomplete stabilization process lasts'about 405 minutes Therefore, in' accordance with tbeLinVention an improved method of stabilizing the electrical characteristics of anxinductor assemblage has been provided; More particularly, a method'of stabilizing: the: delay of delay lines-of. the lumped parameter typeryemploying. inductors:

wound: on: spaced. magnetic cores has been. indicated.

An advantage of the invention is that delay lines can be-stabilized so that the electrical.characteristics remain constant with a high degree' of precision andthe eflects of later variations dueto usage and agingyminimized or eliminated.

There will nowbe obvious to thoseskilled in the. art many modifications and variations utilizing the principles set forth and realizing many or all of the objects and advantages of the methods described but which do not depart essentially from the spirit of the invention.

What is claimed is:

l. The method of stabilizing the inductance ot'an inductor assemblage comprising the steps of heating the" inductor assemblage from room temperature to a;- temperature in the range of about 90 to about 120 centigrade, then cooling the inductor assemblage to about room temperature, and repeating the heatingandcoob ing steps.

2. The method of stabilizing the delayyof alumped' parameter delay line which includes inductor assemblagesconsisting of coil windings on adjacent magnetic cores separated by non-magnetic spacers comprising the steps of heating the delay line for about twenty minutes to a temperature of about 100 centigrade, cooling the delay 3. The. method of stabilizing the delay of a lumped parameter delay line which includesinductor assemblages consisting of coil windings on adjacent magnetic cores separated by non-magnetic spacers'comp rising the steps of heating the delay line for'about sixty minutes to a temperature of about centigrade, cooling the delay line for about thirty'minutes to room temperature, and

separated by non-magnetic spacers comprising the steps of heating the delay line for about ninety minutes to a temperature of about centigrade, cooling the delay line for about forty-five minutes to a temperature of about 20 centigrade, and then repeating the heating and cooling steps twice.

5. The method of stabilizing the inductance of an inductor assemblage consisting of coil windings on two adjacent magnetic cores separated by a non-magnetic spacer comprising increasing the temperature of the inductor assemblage from' room temperature to a temperature in the range of about 90 to 120 centigrade for a period of time in the range of about 20 minutes to about 90 minutes, then cooling. the inductor assemblage to room temperature.

6. The method of stabilizing the inductance of an inductor assemblage comprising increasing the temperature of the inductor assemblage fromambient temperature to' a temperature in the range of about 90 to about 120, and decreasing the temperature of the inductor assemblage to ambient temperature.

7. The method of stabilizing the delay of a lumped parameter delay line which includes inductor assemblages consisting of coil windings on adjacent magnetic cores separated by non-magnetic spacers comprising the steps of heating the delay line for a period of time in the range. of about 20 to 90 minutes to a temperature in the range of about 90 to 120, cooling the delay line to room temperature, and then repeating the heating and cooling steps.

8. The method of stabilizing. the delay of a lumped parameter delay line which includes inductor assemblages consisting of coil windings on adjacent magnetic cores separated by non-magnetic spacers comprising heating the delay line for'a period of time in the range of 20 to 90 minutes to a temperature in the range of about 90 to 120 centigrade, cooling the delay' line for a period of time in the range of about 20' to about 45 minutes to room temperature, and then repeating the heating and cooling steps.

References Cited in the tile of this patent UNITED STATES PATENTS Dunlap May 19, 1953 

1. THE METHOD OF STABILIZING THE INDUCTANCE OF AN INDUCTOR ASSEMBLAGE COMPRISING THE STEPS OF HEATING THE INDUCTOR ASSEMBLAGE FROM ROOM TEMPERATURE TO A TEMPERATURE IN THE RANGE OF ABOUT 90* TO ABOUT 120* CENTIGRADE, THEN COOLING THE INDUCTOR ASSEMBLAGE TO ABOUT ROOM TEMPERATURE, AND REPEATING THE HEATING AND COOLING STEPS. 